The extracellular concentration of adenosine in the brain increases dramatically during ischemia. AdenosineA2Areceptor is expressed in neurons and glial cells and in inflammatory cells (lymphocytes and granulocytes). Recently, adenosineA2Areceptor emerged as a potential therapeutic attractive target in ischemia. Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia the early massive increase of extracellular glutamate is followed by activation of resident immune cells, that is, microglia, and production or activation of inflammation mediators. Proinflammatory cytokines, which upregulate cell adhesion molecules, exert an important role in promoting recruitment of leukocytes that in turn promote expansion of the inflammatory response in ischemic tissue. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression.A2Areceptors present on central cells and on blood cells account for important effects depending on the time-related evolution of the pathological condition. Evidence suggests thatA2Areceptor antagonists provide early protection via centrally mediated control of excessive excitotoxicity, whileA2Areceptor agonists provide protracted protection by controlling massive blood cell infiltration in the hours and days after ischemia. Focus on inflammatory responses provides for adenosineA2Areceptor agonists a wide therapeutic time-window of hours and even days after stroke.

Ischemia is a multifactorial pathology characterized by different events evolving in time. Immediately after the ischemic insult, primary brain damage is due to the massive increase of extracellular glutamate. Adenosine in the brain increases dramatically during ischemia in concentrations able to stimulate all its receptors, A₁, A_2A, A_2B, and A₃. Although adenosine exerts clear neuroprotective effects through A₁ receptors during ischemia, the use of selective A₁ receptor agonists is hampered by their undesirable peripheral side effects. So far, no evidence is available on the involvement of adenosine A_2B receptors in cerebral ischemia. This study explored the role of adenosine A_2B receptors on synaptic and cellular responses during oxygen and glucose deprivation (OGD) in the CA1 region of rat hippocampus in vitro. We conducted extracellular recordings of CA1 field excitatory post-synaptic potentials (fEPSPs); the extent of damage on neurons and glia was assessed by immunohistochemistry. Seven min OGD induced anoxic depolarization (AD) in all hippocampal slices tested and completely abolished fEPSPs that did not recover after return to normoxic condition. Seven minutes OGD was applied in the presence of the selective adenosine A_2B receptor antagonists MRS1754 (500 nM) or PSB603 (50 nM), separately administered 15 min before, during and 5 min after OGD. Both antagonists were able to prevent or delay the appearance of AD and to modify synaptic responses after OGD, allowing significant recovery of neurotransmission. Adenosine A_2B receptor antagonism also counteracted the reduction of neuronal density in CA1 stratum pyramidale, decreased apoptosis at least up to 3 h after the end of OGD, and maintained activated mTOR levels similar to those of controls, thus sparing neurons from the degenerative effects caused by the simil-ischemic conditions. Astrocytes significantly proliferated in CA1 stratum radiatum already 3 h after the end of OGD, possibly due to increased glutamate release. A_2Breceptor antagonism significantly prevented astrocyte modifications. Both A_2B receptor antagonists did not protect CA1 neurons from the neurodegeneration induced by glutamate application, indicating that the antagonistic effect is upstream of glutamate release. The selective antagonists of the adenosine A_2B receptor subtype may thus represent a new class of neuroprotective drugs in ischemia.

Recently, studies have focused on the antihyperalgesic activity of the A₃ adenosine receptor (A₃AR) in several chronic pain models, but the cellular and molecular basis of this effect is still unknown. Here, we investigated the expression and functional effects of A₃AR on the excitability of small- to medium-sized, capsaicin-sensitive, dorsal root ganglion (DRG) neurons isolated from 3- to 4-week-old rats. Real-time quantitative polymerase chain reaction experiments and immunofluorescence analysis revealed A₃AR expression in DRG neurons. Patch-clamp experiments demonstrated that 2 distinct A₃AR agonists, Cl-IB-MECA and the highly selective MRS5980, inhibited Ca²⁺-activated K⁺ (K_Ca) currents evoked by a voltage-ramp protocol. This effect was dependent on a reduction in Ca²⁺ influx via N-type voltage-dependent Ca²⁺ channels, as Cl-IB-MECA–induced inhibition was sensitive to the N-type blocker PD173212 but not to the L-type blocker, lacidipine. The endogenous agonist adenosine also reduced N-type Ca²⁺ currents, and its effect was inhibited by 56% in the presence of A₃AR antagonist MRS1523, demonstrating that the majority of adenosine's effect is mediated by this receptor subtype. Current-clamp recordings demonstrated that neuronal firing of rat DRG neurons was also significantly reduced by A₃AR activation in a MRS1523-sensitive but PD173212-insensitive manner. Intracellular Ca²⁺ measurements confirmed the inhibitory role of A₃AR on DRG neuronal firing. We conclude that pain-relieving effects observed on A₃AR activation could be mediated through N-type Ca²⁺ channel block and action potential inhibition as independent mechanisms in isolated rat DRG neurons. These findings support A₃AR-based therapy as a viable approach to alleviate pain in different pathologies.

Differentiation and maturation of oligodendroglial cells are postnatal processes involving specific morphological changes correlated with the expression of stage-specific surface antigens and functional voltage-gated ion channels. A small fraction of oligodendrocyte progenitor cells (OPCs) generated during development are maintained in an immature and slowly proliferative or quiescent state in the adult central nervous system (CNS) representing an endogenous reservoir of immature cells. Adenosine receptors are expressed by OPCs and a key role of adenosine in oligodendrocyte maturation has been recently recognised. As evaluated on OPC cultures, adenosine by stimulating A1 receptors, promotes oligodendrocyte maturation and inhibits their proliferation; on the contrary by stimulating A2A receptors, it inhibits oligodendrocyte maturation. A1 and A2A receptor-mediated effects are related to opposite modifications of outward delayed rectifying membrane K+ currents (IK) that are involved in regulation of oligodendrocyte differentiation. Brain A1 and A2A receptors might represent new molecular targets for drugs useful in demyelinating pathologies, such as multiple sclerosis (MS), stroke and brain trauma.

Ischemic stroke is a leading cause of death and disability worldwide. The only pharmacological treatment available to date for cerebral ischemia is tissue plasminogen activator (t-PA) and the search for successful therapeutic strategies still remains a major challenge. The loss of cerebral blood flow leads to reduced oxygen and glucose supply and a subsequent switch to the glycolytic pathway, which leads to tissue acidification. Carbonic anhydrase (CA, EC 4.2.1.1) is the enzyme responsible for converting carbon dioxide into a protons and bicarbonate, thus contributing to pH regulation and metabolism, with many CA isoforms present in the brain. Recently, numerous studies have shed light on several classes of carbonic anhydrase inhibitor (CAI) as possible new pharmacological agents for the management of brain ischemia. In the present review we summarized pharmacological, preclinical and clinical findings regarding the role of CAIs in strokes and we discuss their potential protective mechanisms.